As a method of easily removing oil stains adhered onto a hydrophilic coating, a method using water washing is widely known. The reason why oil stains can be easily removed by water washing as described above is because water easily infiltrates into the interface between the oil stains and the hydrophilic coating.
A method in which a hydrophilic coating is formed on the surface of a stainless steel plate forming the inner wall of a dishwasher such that adhesion of stains to the surface of the inner wall is prevented, and thus a drying time of dishes can be shortened is known (for example, refer to Patent Literature No. 1).
However, this method has a problem that the durability of the film of the stainless steel plate is insufficient in a case where the film is exposed to an alkaline detergent at a high temperature for a long period of time. The problem of insufficient alkali resistance is attributable to a large amount of a silicic acid component contained in the hydrophilic coating. Since the silicic acid component dissolves in an alkaline detergent solution at a high temperature, the hydrophilic coating cannot withstand long-term use.
As a composition of an alkali-resistant hydrophilic coating, for example, a composition containing zirconium oxide having excellent alkali resistance and a phosphoric acid component having excellent hydrophilicity is considered instead of the silicic acid component. However, even in a case where the durability (alkali resistance) of the hydrophilic coating in a high-temperature alkaline environment in a dishwasher is improved, a problem of impairment of acid resistance is incurred.
Calcium contained in tap water adheres to the inner wall of a dishwasher in a large amount. The calcium is not removed by an alkaline detergent, and thus needs to be dissolved by an acidic detergent and removed. Since calcium is an inorganic substance, even when calcium adheres to the hydrophilic coating, it is difficult to remove the calcium along with oil components, and it is necessary to periodically remove the calcium using an acidic detergent. As described above, there is a problem that calcium adhered to the hydrophilic coating cannot be sufficiently removed.